It is generally known that electric power stations such as power plants and substations use gas-insulated breakers having a construction such that high-voltage conductors are arranged in a sealed tank filled with insulating gas at a pressure of several-atmosphere and the conductors are electrically insulated from the tank. It is also known that gas-insulated vacuum circuit breakers using vacuum valves are used instead of usual kind of gas-insulated switchgears that are usually used.
The vacuum valve has a pair of openable electrodes accommodated in a vacuum tank kept in a vacuum state and a bellows that prevents the vacuum-leakage of the vacuum tank attributable to the opening movement of the movable electrode while permitting the opening movement of the movable electrode. Because of this configuration, when the vacuum valve is arranged in the sealed tank filled with insulating gas, the bellows is subjected to the vacuum pressure of the vacuum tank on one surface thereof and is subjected to the pressure of the insulating gas filled in the sealed tank on the other surface thereof. This produces difference in pressure between the outside and the inside of the bellows, possibly resulting in shortening the mechanical life of that bellows.
For reduction of the magnitude of such pressure difference applying on the bellows, JP 2005-323499 A1 (Patent Literature 1) has proposed a gas insulated switching device. In the proposed switching device, a vacuum tank kept in a vacuum state is separated, by a pair of bellows, from a sealed container filled with insulating gas; an intermediate room is formed between such pair of bellows; and such intermediate room is pressurized at an intermediate pressure condition between the vacuum pressure of the vacuum tank and the pressure of the insulating gas filled in the sealed container; or instead of providing such intermediate room, the other side of the bellows that holds the degree of vacuum of the vacuum tank is made open to the atmosphere.
JP 06-208820 A1 (Patent Literature 2) has proposed a gas insulated vacuum circuit breaker. In the proposed circuit breaker, an intermediate room is provided on the other side of the bellows that holds the degree of vacuum of the vacuum tank, and such intermediate room is pressurized at an intermediate pressure condition between the vacuum pressure of the vacuum tank and the pressure of the insulating gas filled in the sealed tank.
However, use of a paired bellows in the conventional gas insulated vacuum circuit breaker causes a vacuum valve to be expensive. It is therefore preferable to reduce the difference in pressure between pressures applying on both surfaces of the bellows by use of one bellows. On the other hand, opening the other side of the bellows directly to the atmosphere as Patent Literature 1 describes invites ingress of moisture from the atmosphere and consequently the creeping dielectric strength of the insulator, for example, will be lowered.
In the meantime, when an intermediate room is provided on the other side of the bellows like an example as described in Patent Literature 2, a current collector, which maintains electrical contact with a movable conductor shaft, is installed in the intermediate room. This configuration requires that the current collector should be positioned within the intermediate room over the full-stroke of the switching movements of the movable electrode and the movable conductor shaft. This causes the intermediate room to enlarge greatly its size in the direction of the switching movements and, as a result of this, the gas-insulated vacuum circuit breaker becomes a large apparatus.
An attempt to reduce the size of the intermediate room in the direction of the switching movements means that the movable conductor shaft, the surface of which is roughened or scratched due to sliding contact with the current collector, should pass a gastight holding part provided for prevention of the leakage of the insulating gas filled in the sealed tank, which accommodates the vacuum valve, into the intermediate room. In this construction, there is a risk of a small leakage occurring intermittently or continuously over a long period of time through the gastight holding part attributable to the switching movements of the vacuum circuit breaker. With this case, the internal pressure of the intermediate room gradually rises and reaches an abnormal pressure value inviting a risk of an increased pressure difference between the inside and the outside of the bellows.
An object of the present invention is to provide a gas-insulated vacuum circuit breaker that is configured so that the rise in the pressure in a medium-pressure space, which is provided for the lowering of pressure difference between inner and outer pressure value applying on the bellows, attributable to a small leakage at the airtight holding part will not reach the abnormal pressure value.